Family Description: Aquatic or marsh herbs, usually perennial. Leaves alternate or basal, sheathing at the base. Flowers hermaphrodite or unisexual, usually bracteate and verticillate in pedunculate umbels, racemes or panicles, occasionally long-pedunculate in leaf-axils. Sepals 3. Petals 3, usually larger than the sepals, often fugacious or deciduous. Stamens 3-numerous, with elongated filaments; anthers opening by longitudinal slits. Carpels 3-numerous, spirally arranged or whorled, free or connate at the base; ovules 1-many; styles apical or subventral. Fruit a group of achenes, drupelets or follicles; seeds without endosperm; embryo horseshoe-shaped.

Family Description: Aquatic or marsh herbs; stems stout. Leaves basal, alternate, linear, sheathing at the base. Flowers hermaphrodite or unisexual, ebracteate, in pedunculate racemes or spikes. Perianth of 2-6 sepaloid segments. Stamens 3 or 6, often adnate at the base to the perianth-segments; anthers sessile, 2-thecous, opening by longitudinal slits. Carpels 3 or 6, free or connate, each with 1 ovule; styles short or stigmas sessile. Fruit apocarpous or syncarpous, the fruitlets separating when ripe; seeds with or without endosperm; embryo straight.

Family Description: Perennial, submerged marine herbs. Rhizome creeping, with long internodes, bearing at each node 2 or more unbranched roots and a leaf with a short shoot in its axil. Short shoots with several distichous, linear leaves with compressed, sheathing bases. Flowering stems lateral or terminal. Inflorescence enclosed in the sheathing base of a spathe. Flowers with 1 dorsifixed, sessile stamen and 1 unilocular ovary with a style and 2 long, filiform stigmas, arranged alternately on the margin of the flattened axis; perianth absent. Pollen-grains filiform. Ovule 1.